With the decline in the availability of high aluminum content ores throughout the world and a desire to make use of lower assay domestic clays numerous processes have been suggested and developed for the extraction of aluminum values from such "low" assay clays. Of particular interest in recent years have been the so-called "acid" extraction techniques. According to these techniques, the clay is treated with an acid, usually nitric, hydrochloric or sulfuric, to extract the aluminum values which are subsequently purified and refined according to a variety of conventional or novel methods.
In any such extraction process the first step is to place the clay in a condition such that it can be treated with an appropriate acid to yield the aluminum values while leaving the clay behind. Numerous clay preparation methods have been described and are generally well known to those skilled in this art.
With the evolution of the fluidized or expanded bed and the development of extraction techniques using this operation, the clay preparation step becomes even more crucial to the development of successful commercial clay extraction techniques.
Most alumina digestion operations described in the literature comprise the co-current digestion, over a substantial period of time, of powdered or coarse-ground calcined kaolin at atmospheric or superatmospheric pressure, followed by a large number of stages of counter-current decantation washing of the solids separated from the product liquor. Even when using coarsely-ground calcined clay from which fines have been separated before leaching, the requirements for maintaining the co-currently digesting slurry in a mixed condition, and for obtaining good mixing between the settled solids and wash liquors in the countercurrent decantation washing stages, are impossible to fulfill without the generation of excessive quantities of very fine silicious slimes of around 10 micron and down particles. The slimes settle very slowly, and only to relatively dilute slurries. Large amounts of water, that must be subsequently evaporated, and many washing stages are required to obtain washed slimes which still may contain excessive quantities of acids. However, production by the process herein described of size-tailored pellets, with all the initially-induced edges, corners and protruding areas rounded off toward the spherical shape, permits practice of counter-current digestion and washing methods that avoid the aforesaid washing difficulties and which also can provide a product liquor containing dissolved alumina in excess of stoichiometric requirements. Such pelletizing when using nitric acid leaching permits use of an essentially zero-energy-requirement process for the removal of iron and silica values to levels sufficiently low that the final specifications with regard to these elements can be obtained in a crystallization process.
Many methods, mostly expensive, are well known in the art for preparing pellets of kaolin clay which can then be calcined to provide feed for digestion. We have used in the early stages of our investigations cylindrical extrusions produced by forcing properly-moistened clay through sized holes in a die, and round balls produced by agglomeration of dried, ground clay with water on a pelletizing disk.